1. Field of the Invention
This invention relates to a driving circuit utilized in a television receiver, especially to a driving circuit with a PWM (pulse width modulation) method, for example, a vertical deflection driving circuit, and a sound amplifying circuit using a PWM method.
2. Description of the Related Art
FIG. 5 is an outline of the vertical deflection driving circuit of the PWM method. A sawtooth wave voltage Vin for a vertical deflection is inputted to a positive input terminal (+) of a comparator 2 from an input terminal 1, and a sawtooth wave with frequency f is supplied to a negative input terminal (−) of the comparator 2 as a sampling signal (also known as a carrier signal).
As a result, a PWM signal with a pulse width corresponding to the level of the input sawtooth wave voltage Vin is acquired from the comparator 2 as seen from FIG. 6. The amplitude of the PWM signal is level-shifted by a level adjustment circuit 3 from Vcc1 (5V) to Vcc2, which is an enough voltage for driving output MOS transistors TR1 and TR2. Then, the level-shifted signal is supplied to a driving transistor circuit 4 comprising the output MOS transistors TR1 and TR2.
A pumping-up circuit 5 supplies a driving source voltage to the driving transistor circuit 4. The pumping-up circuit 5 turns on during a retracing period Tr and supplies the voltage of 2 Vcc to the driving transistor circuit 4. Since the transistor TR2 is off and the transistor TR1 is set to be on during the retracing period Tr, the output signal of the driving transistor circuit 4 is 2Vcc. On the other hand, a switching signal, on which a pulse width modulation has been performed, is outputted from the driving transistor circuit 4 during a scanning period Tt. Therefore, the waveform of the output signal from the driving transistor circuit 4 takes the shape shown in FIG. 7A.
Then, the output signal of the driving transistor circuit 4 is taken out from an output terminal 7 through a low pass filter 6. The output from the low pass filter 6 is supplied to a vertical deflection yoke coil not shown in the figure. The waveform of the output from the low pass filter 6 is shown in FIG. 7B. The PWM signal is reproduced as a waveform similar to the waveform of the inverted input sawtooth wave Vin when passing through the low pass filter 6.
With the vertical deflection driving circuit with the abovementioned configuration or a sound amplifying circuit with the same configuration, it is necessary to reduce the switching frequency component fsw by the low pass filter 6 before inputting the switching frequency component fsw of the output MOS transistors TR1 and TR2 to the vertical deflection yoke coil or the speaker. This is because if the switching frequency component fsw is not reduced enough, a raster scan 50 of a television screen 100 shows wave-like distortions as seen from FIG. 8.
The peak of the wave in the raster scan 50 corresponds to the high output of the driving transistor 4 and the valley of the wave in the raster scan 50 corresponds to the low output of the driving transistor circuit 4. When the driving circuit with the PWM method is employed, the peak and valley of the wave in the raster scan 50 are gradually crumbled along with the change in the duty of the PWM signal. There are, for example, 525 raster scans on a television screen. Only a part of the raster scan is shown in FIG. 8.
In order to reduce the switching frequency component fsw, the ability of the low pass filter 6 may be enhanced. However, this may lead to the enlarged size of coil and capacitor as well as the high cost. Although the switching frequency may be ascended for the same purpose, there is a limit for this approach due to the characteristics of the output MOS transistors TR1, TR2 and other elements.